This invention relates to ink jet printing systems, and deals more particularly with an improved head for use in such a system for causing the ejection of ink drops.
The ink drop ejecting head of this invention may be used in various types of ink jet printing systems, but it is particularly well-suited for drop-on-demand systems, and is also particularly well-suited for use in systems for producing large scale graphics, such as billboards, intended for outdoor or other rugged service, where it is desired to have the head eject drops of relatively large volume in comparison to the volume of drops ejected by more conventional heads and where it is desired to have the ink be one which is relatively heavily pigmented and relatively viscous in comparison to inks more conventionally used with ink jet printing heads, the ink therefore having characteristics which might perhaps suggest its being referred to as a paint rather than an ink.
One object of the invention is therefore to provide an ink drop ejecting head of relatively large size capable of ejecting relatively large volume drops of heavily pigmented relatively viscous ink over an acceptable frequency range, say of 0 to 1 kilohertz or more, which ink drop ejecting head may be readily disassembled for repair and cleaning, and which ejecting head also has a nozzle forming the ejection port which nozzle is readily removable from the remainder of the head for cleaning or replacement.
In drop-on-demand ink jet printing systems the frequency of actuation of an ejecting head varies over a wide range since at different times in the scanning of the head relative to the ink drop receiving surface the head may be expected to print only a very few drops or a maximum number of drops or any number of drops therebetween. To achieve proper supply of ink to the ink chamber of the head a positive static pressure is often applied to the ink supply. As a result of this, and sometimes for other reasons in the case of ejection heads which are not supplied with ink at a positive static pressure, when the head is actuated at low frequencies, that is when the period between successive actuations is relatively long, ink will tend to ooze from the ejection port and form an external blob covering the port. On the other hand, at higher actuation frequencies no blob may appear. When a blob is covering the ejection port upon the actuation of the head the ejected ink drop has to shoot through the blob to reach the receiving surface. This has the effect of reducing the velocity of the ejected drop in comparison to the velocity it would have if no blob were present. Also, in shooting through a blob an ejected drop may have its shape varied in comparison to the shape it would have if no blob were present and also small satellite drops may be formed which may reach the receiving surface and have an undesired effect. Preferably, the ejected drops should have a uniform size, shape and velocity regardless of the actuation frequency of the head, but the fact that a blob of ink may cover the ejection port at some actuation frequencies and not at others, or may be of different sizes depending on the actuating frequency militates against the achievement of such uniformity of drop characteristics independent of actuating frequency.
A further object of the invention is therefore to provide an ink drop ejecting head including a means for eliminating or reducing the size of any ink blob which may tend to form externally over the ejection port, thereby through the elimination or reduction of such blob achieving more uniform ejected drop characteristics over the full range of head actuating frequencies.
Other objects and advantages of the invention will be apparent from the following description of preferred embodiments and from the accompanying drawings.